A body suit that is flexible, but upon a hard impact becomes rigid and impenetrable.

How could a man fall from the sky and not be killed?

"It was my impact suit," Purple said. He took a step forward and thumped himself hard in the belly with this fist. His belly was big and soft, so the blow should have made him wince. It did not. I thought for a moment that Purple had become rigid as stone.

"My impact suit," he repeated. "Normally it flows like cloth, but under a sharp blow it becomes a single rigid unit. Lant, you remember that a boy threw a spear at me in your village."

"I remember. You were not hurt."

"The suit is skin tight. With the hood up it covers all of me but my eyes and mouth, and of course it holds my shape. It saved my life."

...Louis ran along the fence toward the walkway. The soldiers were close... The roar was disconcerting, even terrifying. A slug slammmed into Louis's ankle; the suit went rigid, and he rolled like a tumbled statue, picked himself up, and ran again. As two soldiers threw themselves at him, he swung over the fence and dropped...

Louis landed flat on his face, in an impact suit gone rigid as steel. The form-fitting coffin supported him, but he ess still stunned...

Although Gerrold and Niven share authorship of the book, the impact suit is Niven's idea; see Flexible Armor Suit from The Soft Weapon, a short story published in 1967.